IMO;
Charizard, Sceptile, Pikachu, Heracross, Snorlax, Infernape.
Charizard has won countless battles (As has Pikachu) but it clearly has a lot more power. Sceptile was strong to begin and then it defeated Darkrai, albeit with help from Gible and Torkoal but it still defeated it. Pikachu because it's well... Pikachu spam Thunderbolt etc. It did take down Latios, though. Heracross seemed pretty god damn strong in the battles it took part in. I loved watching it's battles in the Sinnoh league. Snorlax seemed strong but it never really got any battles to truly show off it's power -- Infernape gets last place because the former mentioned are just stronger. I'd maybe swap Infernape with Heracross and then Snorlax with Heracross. idk.